In an optical communication field and an optical measurement field, an optical modulator, in particular, an optical modulator in which an optical waveguide and a modulation electrode which modulates light waves propagating through the optical waveguide are provided in a substrate is frequently used. In recent years, a polarization-combining type multi-level modulator enabling optical communication exceeding 100 Gbps has also been used. In such an optical modulator, a structure is made in which a plurality of optical modulator units each having a Mach-Zehnder type optical waveguide are integrated. For this reason, a configuration of a modulation part to drive at low voltage (apart in which an electric field formed by a modulation electrode acts on an optical waveguide, and also referred to as an “active region”) is required.
In general, in order to realize a low drive voltage, it is known that the strength of an electric field which is applied to an optical waveguide is increased by narrowing the distance between a signal electrode and a ground electrode which configure a modulation part. On the other hand, in an optical modulator having a traveling electrode, in order to realize a broadband characteristic, it is necessary to match the velocity of the propagating light of an optical waveguide in a modulation part and the velocity of a modulation signal propagating through a modulation electrode. Usually, in order to match the velocity of the propagating light and the velocity of the modulation signal, it is necessary to increase an electrode thickness. In this manner, in a case of increasing the electrode height, the impedance of a RF line (a modulation electrode) of the modulation part becomes lower and becomes even lower than 50Ω which is general impedance of an external signal circuit.
In a case where the impedance of the modulation part is different from the impedance of a signal source or a modulator driver which is an external signal circuit of the optical modulator, reflection of an electric signal which is input to the optical modulator is generated due to impedance mismatch, thereby causing degradation of a signal or an increase of drive voltage. For this reason, a technique of forming an impedance matching line on a modulator device substrate, as shown in Patent Literature No. 1, or a technique of improving impedance matching by inserting a resistor in series into a line by using a relay board or the like, as in Patent Literature No. 2, is known.
In a case where the impedance of the modulation part is low, even in a case of improving a signal reflection characteristic S11 by forming the impedance matching line by using the technique disclosed in Patent Literature No. 1, in a low-frequency area in which a sufficient line length is not secured, and thus impedance matching is difficult, the S11 characteristic is generally determined by connector impedance on the signal input side set to impedance equal to that of an external signal circuit, and termination impedance of a termination circuit. For example, in a case where a connector is 50Ω and a termination circuit is 25Ω, the amount of reflection of −9.5 dB is obtained.
Further, in a case of improving impedance matching by inserting a relay board having a resistor series-connected to a signal line into the front section of a low-impedance line which is a modulation part, as in Patent Literature No. 2, electric power is consumed by the resistor, and thus a problem in which the amplitude of a signal which is input to the modulation part is reduced occurs. Further, if a frequency becomes higher, the influence of a line length of the modulation part compared to the signal wavelength becomes non-negligible, and synthetic impedance on the modulator side when viewed from a connector greatly deviates from the sum of matching resistance and termination resistance, and thus there is also a problem in which impedance matching cannot be taken.